QList<T>::append(const T&) modifies the operand if it's Qt container

QList<T>::append(const T&) call shares internal data used by T if it's some sort of Qt container, thus making it possible to silently invalidate its iterators on next call to T::end() or any other modifying member function.

There are two tests in attachment, for QList<QList<int>> and QList<QString>.